Disrupted cortical map and absence of cortical barrels in growth-associated protein (GAP)-43 knockout mice.

There is strong evidence that growth-associated protein (GAP-43), a protein found only in the nervous system, regulates the response of neurons to axonal guidance signals. However, its role in complex spatial patterning in cerebral cortex has not been explored. We show that mice lacking GAP-43 expression (-/-) fail to establish the ordered whisker representation (barrel array) normally found in layer IV of rodent primary somatosensory cortex. Thalamocortical afferents to -/- cortex form irregular patches in layer IV within a poorly defined cortical field, which varies between hemispheres, rather than the stereotypic, whisker-specific, segregated map seen in normal animals. Furthermore, many thalamocortical afferents project abnormally to widely separated cortical targets. Taken together, our findings indicate a loss of identifiable whisker territories in the GAP-43 -/- mouse cortex. Here, we present a disrupted somatotopic map phenotype in cortex, in clear contrast to the blurring of boundaries within an ordered whisker map in other barrelless mutants. Our results indicate that GAP-43 expression is critical for the normal establishment of ordered topography in barrel cortex.

BLyS: member of the tumor necrosis factor family and B lymphocyte stimulator.

The tumor necrosis factor (TNF) superfamily of cytokines includes both soluble and membrane-bound proteins that regulate immune responses. A member of the human TNF family, BLyS (B lymphocyte stimulator), was identified that induced B cell proliferation and immunoglobulin secretion. BLyS expression on human monocytes could be up-regulated by interferon-gamma. Soluble BLyS functioned as a potent B cell growth factor in costimulation assays. Administration of soluble recombinant BLyS to mice disrupted splenic B and T cell zones and resulted in elevated serum immunoglobulin concentrations. The B cell tropism of BLyS is consistent with its receptor expression on B-lineage cells. The biological profile of BLyS suggests it is involved in monocyte-driven B cell activation.

The binding epitopes of neurotrophin-3 to its receptors trkC and gp75 and the design of a multifunctional human neurotrophin.

Survival and maintenance of vertebrate neurons are influenced by neurotrophic factors which mediate their signal by binding to specific cell surface receptors. We determined the binding sites of human neurotrophin-3 (NT-3) to its receptors trkC and gp75 by mutational analysis and compared them to the analogous interactions of nerve growth factor (NGF) with trkA and gp75. The trkC binding site extends around the central beta-strand bundle and in contrast to NGF does not make use of non-conserved loops and the six N-terminal residues. The gp75 epitope is dominated by loop residues and the C-terminus of NT-3. A novel rapid biological screening procedure allowed the identification of NT-3 mutants that are able to signal efficiently through the non-preferred receptors trkA and trkB, which are specific for NGF and BDNF respectively. Mutation of only seven residues in NT-3 resulted in a human neurotrophin variant which bound to all receptors of the trk family with high affinity and efficiently supported the survival of NGF-, BDNF- and NT-3-dependent neurons. Our results suggest that the specificity among neurotrophic factors is not solely encoded in sequence diversity, but rather in the way each neurotrophin interacts with its preferred receptor.

Regulation of neurotrophin receptor expression during embryonic and postnatal development.

Members of the NGF family of proteins act as neurotrophic agents for defined populations of peripheral and central neurons during embryonic and postnatal development. We have studied the presence of receptors for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 and -4/5 (NT-3, NT-4/5) by cross-linking radioiodinated neurotrophins to specific cell surface receptors. We have identified neurotrophin receptors representing full-length TrkB and TrkC and their truncated forms (lacking a functional cytoplasmic kinase domain) in neuronal as well as in non-neuronal tissues. During chicken embryonic and early postnatal brain development, expression of full-length TrkB and TrkC proteins preceded the onset of the truncated forms of these receptors. A similar pattern was also observed in mouse embryonic and early postnatal brain. The relative levels of neurotrophin receptors in the basal forebrain and in the hippocampus did not change significantly with age in mice. High levels of receptors for the three neurotrophins were detected in the nigrostriatal system. Full-length TrkB and TrkC receptors were found in chicken and rat embryonic ventral spinal cord, as well as on purified motoneurons. Again, truncated TrkB appeared significantly later than the full-length form on spinal motoneurons. In chicken embryonic retina and optic tectum we detected full-length TrkB and TrkC; however, the optic tectum also expressed large amounts of the truncated form of TrkB. TrkC but not TrkB was detected in chicken embryonic skeletal muscle, suggesting that NT-3 may have a novel function in this tissue. The presence of neurotrophin receptors in a wide variety of embryonic and postnatal tissues underlines the significant role of BDNF, NT-3, and NT-4/5 in embryonic and postnatal development. The regulation of the ratio of full-length versus truncated neurotrophin receptors may play an important role in the development, maturation, and maintenance of various neuronal networks.

Distribution of trkB tyrosine kinase immunoreactivity in the rat central nervous system.

Recent evidence suggests that trkB tyrosine kinase is a high affinity receptor for brain-derived neurotrophic factor (BDNF). BDNF can act as a survival factor for several neuronal subgroups and its mRNA is distributed widely throughout the central nervous system. However, the functional targets of BDNF are poorly defined. We have used immunochemical and immunohistochemical techniques to determine the regional distribution and cellular localization of trkB tyrosine kinase-like immunoreactivity. The staining pattern indicates that the trkB-like antigen is widely distributed and present within both glia and neurons. Astrocytes were the most intensively labelled but many neuronal populations were also stained. In some regions including brain stem, spinal cord, hippocampus and diagonal band of Broca, neurons were stained at varying intensities. In other areas such as the cortex of the forebrain and amygdaloid nucleus, the stain was intense but diffuse, preventing positive identification of the cell types involved. Immunoblot results indicated two separate protein bands in all brain and spinal cord regions examined, of molecular weights 145 and 85 kDa, respectively. These findings aid the definition of neuronal and glial subpopulations of the central nervous system that may utilize BDNF.

The rat trkC locus encodes multiple neurogenic receptors that exhibit differential response to neurotrophin-3 in PC12 cells.

Members of the Trk tyrosine kinase family have recently been identified as functional receptors of the NGF family of neurotrophins. Here we show the rat trkC locus to be complex, encoding at least four distinct polypeptides. Three of the encoded polypeptides are full-length receptor tyrosine kinases that differ by novel amino acid insertions in the kinase domain. A fourth protein is a truncated receptor that lacks the catalytic domain. Tyrosine phosphorylation, cross-linking, and ligand binding assays indicate that TrkC receptors interact with NT-3 and not with the related neurotrophins NGF, BDNF, xNT-4, or hNT-5. Furthermore, high and low affinity NT-3-binding sites are associated with the TrkC receptors. Stable and transient expression of TrkC receptors in PC12 cells indicates that the neurite outgrowth response elicited by NT-3 is dramatic in receptors lacking the novel kinase insert (gp150trkC) but absent in receptors containing the 14 amino acid insert in the kinase domain (gp150trkC14). These data suggest that the trkC locus encodes receptors that may be capable of mediating different biological responses within the cell. This could have important implications in understanding the role of neurotrophins in the development of the vertebrate nervous system.

Sequence and expression of caveolin, a protein component of caveolae plasma membrane domains phosphorylated on tyrosine in Rous sarcoma virus-transformed fibroblasts.

Caveolae are flask-shaped plasma membrane invaginations abundant in endothelium and muscle but may be present in all cells. They contain a filamentous coat material thought to be important in their structure and function. Recent studies have demonstrated that a 22-kDa protein (caveolin) phosphorylated on tyrosine in Rous sarcoma virus-transformed chicken fibroblasts is a component of the caveolae coat on the inner aspect of the membrane. We now report the deduced protein sequence of chicken caveolin derived from cDNA PCR products and genomic DNA clones. Caveolin is a unique protein of 178 amino acids and displays little sequence similarity to other proteins in the GenBank data base. Hydrophobicity predictions indicate an unusual 40-amino acid hydrophobic region near the C terminus that may be used to anchor the protein to the membrane. When chicken caveolin was expressed in mouse 3T3 cells and detected by immunofluorescence microscopy, the typical caveolae pattern was observed. This includes brightly fluorescent membrane patches in many cases concentrated at the margin of cells and in arrays. Caveolae may be distinct from other membrane domains due at least in part to caveolin.

Evidence for unequal crossing over in the evolution of the neurofilament polypeptide H.

Two allelic forms of the rabbit neurofilament protein H, designated H1 and H2, differ by approximately 6% in their electrophoretic mobilities. We have used techniques of peptide and nucleic acid analysis to determine that this difference is located in the central portion of the COOH-terminal tail, a region of the H protein that can project from the filament, and may form cross-bridges. This region comprises tandem amino acid motifs containing the sequence KSP, in which the serine residues are sites of phosphorylation. The sequence of this repetitive region of H2 is 132 nucleotides (44 amino acids) shorter than H1 and differs significantly in the arrangement of the repeated motifs. The difference suggests that both forms have evolved from a common ancestor over the past several million years by the process of unequal crossing over and that this process figured importantly in generating the repetitive region. In addition, the results indicate that this region is functionally redundant and that the length and structure of the tail of rabbit H resemble H from other mammalian species more closely than was suggested by a previously reported cDNA, on account of a single nucleotide difference.

The Trk family of tyrosine kinases: receptors for NGF-related neurotrophins.

Neurotrophins are known to have important functions in the survival of embryonic and adult subpopulations of neurons. The identification of Trk family RTKs as receptors for NGF-related neurotrophins indicates phosphotyrosine-mediated signal transduction as a principal mechanism for neurotrophin signaling. Previous trk and trkB expression studies (Klein et al. 1989, 1990b; Martin-Zanca et al. 1990) and more recent studies with trkC (L. Tessarollo et al., in prep.) provide important clues about function. Thus, trkB and trkC expression in motor neurons and in many nonneuronal cells suggests that these cells are targets for neurotrophin action in vivo, even though this has not been demonstrated in the classic in vitro survival assays. Expression of trkB and trkC in nonneuronal cells implies that these receptors may act in additional aspects of organogenesis and development. Current approaches to assay Trk receptor and neurotrophin function will be complemented by further studies in the living organism. Transgenic approaches aimed at ectopic expression and at interfering with normal receptor function should provide additional insights. Finally, reverse genetic approaches using targeted mutation of Trk receptors in embryonic stem cells (Stanton et al. 1992) will allow assessment of critical receptor requirements and provide powerful reagents for studying nervous system development and function.

Sequence of the rabbit neurofilament protein L.

In the course of screening a rabbit brain cDNA library with a probe for the H neurofilament protein, we identified a neurofilament L-cDNA. Its nucleotide sequence is 88% identical to that of human, indicating that L is highly conserved among species. The similarities between the sequences of L from rabbit and mouse suggest that the species-specific accumulation of neurofilaments that occurs in rabbit during aluminum intoxication is not a consequence of the primary structure of L.

The neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 are ligands for the trkB tyrosine kinase receptor.

Neurotrophic factors are essential for neuronal survival and function. Recent data have demonstrated that the product of the tyrosine kinase trk proto-oncogene binds NGF and is a component of the high affinity NGF receptor. Analysis of the trkB gene product, gp145trkB, in NIH 3T3 cells indicates that this tyrosine kinase receptor is rapidly phosphorylated on tyrosine residues upon exposure to the NGF-related neurotrophic factors BDNF and NT-3. Furthermore, gp145trkB specifically binds BDNF and NT-3 in NIH 3T3 cells and in hippocampal cells, but does not bind NGF. Thus, the trk family of receptors are likely to be important signal transducers of NGF-related trophic signals in the formation and maintenance of neuronal circuits.

Purification of the growth-associated protein GAP-43 by reversed phase chromatography: amino acid sequence analysis and cDNA identification.

GAP-43 is a neuronal phosphoprotein. Increased synthesis and axonal transport of GAP-43 has been associated with axon growth, and altered phosphorylation of GAP-43 has been associated with changes in synaptic efficacy. Here we report a rapid and effective procedure employing reverse-phase HPLC for the purification of GAP-43 from rat brain. To characterize the protein purified by this procedure, we generated proteolytic fragments and determined their amino acid sequences. These directly determined sequences, corresponding to 56% of the GAP-43 amino acids, confirm recently reported sequences deduced from the nucleotide sequences of cDNAs. Using oligonucleotide probes constructed according to these amino acid sequences, we identified GAP-43 cDNAs in a library prepared from neonatal rat superior cervical ganglion cells. One of these cDNAs was 1.1 kB in size; it hybridized specifically with a 1.5 kB RNA from brain, but not from liver, and contained the entire coding sequence for GAP-43. This cDNA differed from recently reported cDNAs in its 3' untranslated region.

Posttranslational modification of neurofilament polypeptides in rabbit retina.

Three polypeptides that compose neurofilaments, designated H, M, and L, are synthesized in the cell bodies of neurons and subsequently conveyed down their axons by the process of slow axonal transport. The axonal form of H, which is a component of the cross bridges between the neurofilaments, is antigenically different from the form in the cell bodies and dendrites. To understand how this special form of H is directed to the axon, and more generally how intracellular differentiation is established and maintained by the selective delivery of different molecular species to different compartments of a cell, we have studied the events that occur immediately after the synthesis of the three neurofilament polypeptides in the retinas of rabbits. We observed that H and M are synthesized in the retina as precursor polypeptides, EH and EM, that migrate markedly faster on SDS polyacrylamide gels than their mature axonal forms. The maturation of these precursors requires more than one day and appears to involve their phosphorylation. Only the electrophoretically mature forms appear in the axons of the retinal ganglion cells in the optic nerve. We consider the following interpretation of these observations. Shortly after they are translated in the cell body, the neurofilament polypeptides become phosphorylated at multiple sites. However, only after they have moved a distance of several hundred micrometers down the axon, H and M are phosphorylated at additional sites, causing their conformation or binding properties to change. This change, which is reflected in the reduction of their electrophoretic mobility and the appearance of new antigenic determinants, may function to alter the H-mediated crossbridges and produces the morphological and structural properties of the neurofilament lattice that is characteristic of axons.

Activities of transport enzymes located in the plasma membranes of corneal endothelial cells.

Plasma membranes from bovine corneal endothelial cells have been prepared for assay of ion transport enzymes (ATPase, EC 3.6.1.8). The membrane were judged to be highly purified by protein recovery, electron microscopy, and assay of enzyme markers. Sodium-potassium-stimulated ATPase was more than six times more active in the membranes (6.7 mumoles phosphate released/mg protein/30 min) than the whole homogenates (1.1 mumoles phosphate released). However, no increase in activity was seen with anionic ATPase of membranes (2.5 mumoles phosphate released with bicarbonate stimulation) vs. whole homogenates (3.0 mumoles phosphate released; insignificant difference). In contrast, the bicarbonate-stimulated ATPase activity of a mitochondrial preparation from the same cells was 10.6 mumoles phosphate released (p less than 0.001). It is suggested that anionic ATPase has no direct role in deturgescence as part of and endothelial cell "pump."

The effects of glutathione and adenosine on plasma membrane ATPases of the corneal endothelium. An hypothesis on the stimulatory mechanism of perfused glutathione upon deturgescence.

Reduced glutathione (0.3 mM) stimulates the activity of sodium-potassium activated ATPase (Na+K+ATPase) by 54% in plasma membranes prepared from bovine corneal endothelial cells. Oxidized glutathione, however, has no effect on Na+K+ATPase activity in the same tissue, although it does inhibit magnesium activated ATPase (Mg++ATPase) by approximately 30%. Adenosine neither stimulates nor inhibits either Na+K+ATPase or Mg++ATPase in these plasma membranes. It is postulated that the stimulatory effect of glutathione on deturgescence stems from the direct reaction of the reduced form of the tripeptide on sulfhydryl groups located on plasma membranes of corneal endothelial cells. It is highly probable that these sulfhydryl groups are part of the Na+k+ATPase complex itself.